The effect of using different values for the effect-site equilibrium half-time on the prediction of effect-site sevoflurane concentration: a simulation study.

Unlabelled: We have developed a predictive display that allows effect-site concentration (Ceff) to be used as a target for administration of inhaled anesthesia. Ceff is dependent on the half-time for plasma effect-site equilibrium [t 1/2 1/2(ke0)]. The t 1/2(ke0) used in the predictive display is fixed and may differ from that in the patient. We wished to explore the effect of this difference on predictions of Ceff. In a computer simulation, fresh gas flow and vaporizer settings required to achieve a predefined time profile for Ceff were determined for t 1/2(ke0) of 2.5, 3.5, and 5 min. The end-tidal values for each simulation were used to recalculate Ceff with each t 1/2(ke0). The maximal deviation at predetermined points, measures of global fit, and the delay in "recovery" were calculated. With a predictive display t 1/2(ke0) of 3.5 min, the maximal error in Ceff was 0.18 vol%, occurring during the wash-in phase and disappearing within 2-3 min. The difference in time for Ceff to decrease from 1.0 to 0.7 vol% was 1.3 min. Results with a display t 1/2(ke0) of 2.5 min or 5 min and simulated patient t 1/2(ke0) of 5 min or 2.5 min were approximately twice as large. These results suggest that Ceff is relatively insensitive to large (50%-100%) variations in t 1/2(ke0).

Implications: A model-based predictive display to guide effect site targeting of volatile anesthesia is described. The effect of using different values for the rate of transfer of sevoflurane between central and effect site compartments is explored. The results suggest effect site concentration is relatively insensitive to 50-100% variations in half-time for plasma effect-site equilibrium.